Induction of human platelet fibrinogen receptors by epinephrine in the absence of released ADP

The ability of epinephrine to expose platelet fibrinogen receptors independent of released ADP was assessed using aspirin-treated, gel- filtered platelets. Similar to ADP-induced aggregation, platelet aggregation in response to epinephrine was accompanied by fibrinogen binding. Ten micromolar epinephrine induced a maximum number of platelet fibrinogen receptors in the absence of significant 14C- serotonin release. As indicated by Scatchard analysis, receptors exposed by both epinephrine and ADP had similar affinities for fibrinogen, but epinephrine induced approximately 30% fewer receptors than did ADP. This appears to correlate with the lesser degree of primary aggregation observed with this agent. Studies using phentolamine, a specific alpha-adrenergic antagonist, apyrase, or creatine phosphate/creatine kinase indicate that the exposure of platelet fibrinogen receptors by epinephrine was specific for platelet alpha-adrenergic receptor stimulation and was not the result of released ADP.     

Identification of a growth factor for primary murine stroma as macrophage colony-stimulating factor

Knowledge of the stromal microenvironment is crucial for understanding the hematopoietic system. We took advantage of an assay that permits analysis of primary stroma-initiating cells (SICs) on the clonal level, and further characterized SICs and the factors that regulate SICs. Stroma formation in this assay is dependent on a high-molecular-weight factor secreted by the stromal cell line AC3.U. Here we show that this factor is identical to macrophage colony-stimulating factor (M-CSF), and that purified M-CSF is sufficient for induction of stroma formation. M-CSF, isolated from the line AC3.U, as well as from L929 cells and COS cells transfected with an expression vector encoding M- CSF, migrated in two peaks as 160- and 650-kD species after gel filtration. These molecular-weight species encompassed all stroma- inducing activity, and both stimulated macrophage colony formation. Affinity chromatography and blocking studies with antibodies specific for M-CSF and c-fms confirmed M-CSF as the sole factor in the supernatant of the stromal cell line AC3.U that promotes stroma formation. Culture of marrow, for as little as 1 week, depleted M-CSF- dependent SIC while increasing the incidence of replatable, factor- independent SIC. This suggests that culture changes the properties of SICs, perhaps by inducing differentiation into mature stromal cells. Thus, our results show a novel function of M-CSF as an important modulator of stroma formation.